A Polarization‐Modulated Information Metasurface for Encryption Wireless Communications

Abstract Programmable and information metasurfaces have shown great potentials in wireless communications, but there are few reports on encrypted communications. In this paper, a programmable polarization‐modulated (PoM) information metasurface is proposed, which can not only customize arbitrarily linearly polarized reflected waves, but also modulate their amplitudes in real time. Based on this feature, a physical‐level wireless communication encryption scheme is presented and experimentally demonstrated by introducing a meta‐key, which can be encrypted and sent by the programmable PoM information metasurface. To be specific, the key is encoded and concealed into different linear polarization channels, and then modulated and transmitted by the information metasurface at the transmitting end. At the receiving end, the modulated signal can be received and decoded by using a pair of polarization discrimination antennas. A wireless transceiver system is established to verify the feasibility of the scheme. It is shown that, once the meta‐key is obtained, the corresponding encrypted target information that has been sent to the user in advance can be recovered.


S1. Encryption and decryption processes
. Scheme of the encryption process with modulated signals as the meta-key and decryption process with demodulated signals as the meta-key. Photo credit: Hai Lin Wang, Southeast University. Figure S1 shows the scheme of encryption and decryption processes for an image by adopting a meta-key. In the transmitting end, the target image (i.e., lynx) can be decomposed to grayscale images of RGB and encoded as a matrix of binary numbers ( × ), and then the matrix is encrypted by using the meta-key following the principle of simple XOR operation to generate a cipher image. If the user only receives information of cipher image, he/she cannot know the true target image.
Hence, the meta-key should also be encrypted and sent to the user, which is focus of this work and can be achieved by using the proposed programmable PoM information metasurface. Once the user gets the correct meta-key, the real information of target image can be obtained by the XOR operation of the meta-key and encrypted matrix of cipher image. Figure S2. The simulated reflection amplitude and phase of unit element varying with Rdx and Rdy in a wide band of 9-11 GHz. a) Simulated reflection amplitude of x-polarized wave with Rdx=1 to 30 Ω. b) Simulated reflection phase of x-polarized wave with Rdx=1 to 30 Ω. c) Simulated reflection amplitude of x-polarized wave with Rdx=35 to 10000 Ω. d) Simulated reflection phase of x-polarized wave with Rdx=35 to 10000 Ω. e) Simulated reflection amplitude of y-polarized wave with Rdy=1 to 30 Ω. f) Simulated reflection phase of y-polarized wave with Rdy=35 to 10000 Ω. g) Simulated reflection amplitude of y-polarized wave with Rdy=1 to 30 Ω. h) Simulated reflection phase of y-polarized wave with Rdy=35 to 10000 Ω.

S2. Simulated results of amplitude and phase responses of PoM metasurface
Figures S2a-d demonstrate the simulated reflection coefficient (S11) of the unit element varying with Rdx and Rdy in the frequency band of 9 to 11 GHz. When Rdx is increased from 1 to 30 Ω, the reflection amplitude keeps decreasing from about 0.9 to 0 but the reflection phase remains basically unchanged, as shown in Figures S2a and S2b. However, when Rdx is increased continuously from 35 to 10000 Ω, the reflection amplitude keeps increasing from about 0.1 to 1 but the reflection phase also remains basically unchanged, as shown in Figures  the reflected wave will be x polarization, as shown in Figure S3b. Similarly, when resistances of PIN diodes along x and y directions are set to R dx =30 Ω and R dy =10000 Ω, respectively, only the y polarization component is efficiently reflected, while the x polarization component is completely absorbed, and the reflected waves will be y polarization, as shown in Figure S3c. However, when resistances of PIN diodes along x and y directions are set to R dx =10000 Ω and R dy =1 Ω, respectively, both the x and y polarization components are efficiently reflected with the same reflection amplitude, but have a phase difference of 180°, so the reflected wave will be left-handed circularly polarization of incident wave, as shown in Figure S3d. It is worth noting that although only four special cases are demonstrated, the reflected wave with arbitrary polarization ellipticity can be achieved by accurately controlling the amplitude and phase of the x and y reflection components.

S4. Measured results of amplitude and phase responses of PoM metasurface
Figures S4a and 4b illustrate the measured amplitude and phase responses of S11 under the x-polarized incidence as the bias voltage increases from 0 to 0.63 V, where the voltage accuracy can achieve 0.01 V. The results show that the reflection amplitude continuously decreases in a wide band of 9 to 11 GHz, but the reflection phase almost remains unchanged. Figure S4c and 4d illustrate the measured amplitude and phase responses of S11 under the x-polarized incidence as the bias voltage is further increased from 0.64 to 0.83 V, which show that the reflection amplitude continuously increases in a wide band of 9 to 11 GHz, and the reflection phase almost remains unchanged. The similar measurement results for y-polarized incidence can be achieved, as shown in Figures S4e-h.

Figures S6a and S6b show the top and bottom views of the proposed array antenna,
having a total size of 110×110 mm 2 . Twelve square patch elements and the feed circuit are arranged in a mirror-symmetric form, as shown in Figure S6a. The slot lines and slot ring are etched on the back of the antenna, as shown in Figure S6b, where four zero-bias Schottky diodes (SMS7621-040LF) were loaded on the slot ring to realize the double-balanced RF multiplier. A commercial dielectric F4B with a relative permittivity of 2.2, a thickness of 0.5 mm, and a loss tangent of 0.001 is adopted in this design. The dimensions of the parameters are lp=110mm, l1=30mm, l2=20.5mm, l3=5.5mm, w1=9mm, and ls=37.7mm. Figure S6c depicts the measured scattering parameter (S-parameter) S11 of the polarization discrimination antenna, which is below −10 dB in the frequency range of 9.9-10.1 GHz. The simulated radiation pattern of the array antenna at 10GHz is demonstrated in Figure S6, showing that the gain is 13dBi.